Is "master mono" a channel in the Alsa Mixer? If it is, can you add the output of amixer scontrols to the question.
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mikewhateverDec 25 '12 at 17:22

1

The volume keys are handled not by alsamixer but by your desktop environment. Which one?
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CL.Dec 25 '12 at 21:53

Please provide the output of 'amixer scontrols' as @mikewhatever requested. To the best of my knowledge (and testing over numerous platforms) there is no master mono channel in Alsa mixer
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Elder GeekMay 17 '14 at 22:52